The present invention relates to apparatus for use in connection with maps to aid in determining whether there is a line of sight between two spaced points of the terrain which is represented by the map.
Various devices have been proposed for determining whether such articles as rails and pipes extend along a straight line, these generally including a plurality, such as three, of elements which are caused to be attached to or associated with the pipe or rail at three spaced apart points.
In one such apparatus, for use in hanging and lining up shafting, there were provided three posts which were attachable to the shafts, and which depended generally perpendicularly to the shaft. One post carried a sighting telescope which could be adjusted vertically, and other posts contained sighting targets, for sighting through the telescope. When the sighting telescope and targets were in alignment, the shaft was deemed to be extending in a straight line.
Similar organizations were provided in connection with the laying of railway track. In these instances, posts were attached to the tracks at three spaced apart points, one of the posts having a sighting cursor, used to sight across the top of the next two posts which had been mounted on the rail. When the tops of the other two posts were sighted as being in alignment with the cursor, it was thereby determined that the rail was straight.
Other apparatus of interest in connection with the herein disclosed subject matter included various alidades, used for constructing maps, sighting fires from a look-out point, and the like. These usually included a rotatable base, with a pair of posts extending generally perpendicularly to the base, and with cursors movable along the posts. Thus, an angle to the horizontal could be established by lining up a particular terrain feature with the two cursors on the two posts, and thereby the position of this terrain feature from the observation point determined.
None of the known apparatus, however, is suitable for determining, from use thereof in cooperation with a map, whether an intervening terrain feature shown on the map will or will not block the line of sight between two other terrain features represented by the map, and lying on either side of the noted intermediate terrain feature.